Logging tool with a parasitic radiation shield and method of logging with such a tool

Abstract

It is described a logging tool 20 for underground formations surrounding a borehole 14 , comprising an elongated body 21 along a major axis; a collar 22 disposed peripherally around said body 21 having a collar wall defined by an inner and an outer surface; a radiation emitting source 201 arranged to illuminate the earth formation 16 surrounding the borehole; at least one radiation detector 211 arranged to detect radiation reflected by the earth formation resulting from illumination by the source 201 ; at least one source collimation—window 202 and one detector collimation window 212 through which the earth formation is illuminated and radiation is detected; and characterized in that it further comprises at least one radiation shield 30 located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic radiation that has not traversed the outer collar.

1. A logging tool ( 20 ) for underground formations surrounding a borehole ( 14 ), comprising:
 an elongated body ( 21 ) along a major axis; 
 a collar ( 22 ) disposed peripherally around said body ( 21 ) having a collar wall defined by an inner and an outer surface; 
 a radiation emitting source ( 201 ) arranged to illuminate the earth formation ( 16 ) surrounding the borehole; 
 at least one radiation detecttor ( 211 ) arranged to detect radiation reflected by the earth formation resulting from illumination by the source ( 201 ); 
 at least one source collimation window ( 202 ) and one detector collimation window ( 212 ) through which the earth formation is illuminated and radiation is detected; 
 at least one radiation shield positioned so as to eliminate patasitic radiation that has not traversed the outer collar and is located between said emitting radiation source and said radiation detector, said radiation shield having a length along the axis that is less than 80% of the distance between said emitting radiation source ( 201 ) and said radiation detector ( 211 ). 
 
     
     
       2. The tool of  claim 1  further comprising a stabilizer ( 24 ) located at the periphery around the outer collar surface, wherein said stabilizer comprises a stabilizer wall defined by an inner stabilizer surface and an outer stabilizer surface, and wherein the radiation shield is located between said inner collar surface and said outer stabilizer surface. 
     
     
       3. The tool of  claim 1 , wherein another radiation shield ( 31 ) has an annular shape surrounding the detector collimation window ( 212 ) and has preferably a length along the axis, which is less than 40% of the distance between said source and said detector. 
     
     
       4. The tool of  claim 1 , wherein said radiation shield has a thickness in the cross section perpendicular to the major axis, which is less than 40% of the width of the said logging tool at the position of said emitting radiation source ( 201 ). 
     
     
       5. The tool as claimed in  claim 1 , wherein the radiation emitting source is chosen in the list of: chemical radioactive source, an electronic radiation source emitting x-ray, a chemical neutron source or an electronic neutron source. 
     
     
       6. The tool of  claim 1 , wherein said logging tool ( 20 ) is a gamma-ray scattering density measurement tool wherein:
 said emitting radiation source ( 201 ) is a gamma ray source or an x-ray source; 
 a first radiation detector ( 211 ) is a short spaced gamma ray detector spaced axially at a first distance from said; and 
 a second radiation detector ( 221 ) is a long spaced gamma ray detector spaced axially at a second distance, greater than said first distance, from said emitting radiation source. 
 
     
     
       7. The tool of  claim 1 , wherein said logging tool ( 20 ) is a neutron scattering porosity measurement tool wherein:
 said emitting radiation source is a neutron source; and 
 said radiation detector is a neutron detector spaced axially at a first distance from said neutron source. 
 
     
     
       8. The tool as claimed in  claim 1 , wherein said radiation shield is made of high atomic number material. 
     
     
       9. The radiation shield as claimed in  claim 8 , wherein the high atomic number material includes tungsten, depleted uranium, gold or an alloy thereof. 
     
     
       10. The tool as claimed in  claim 1 , wherein said radiation shield is made of components selected from the list consisting of: hydrogenous material, neutron absorbing materials, boron and cadmium. 
     
     
       11. A method for logging a well comprising the steps of:
 lowering a logging tool in a well; and 
 logging well using logging tool; 
 wherein the logging tool comprising:
 an elongated body ( 21 ) along a major axis; 
 a collar ( 22 ) disposed peripherally around said body ( 21 ) having a collar wall defined by an inner and an outer surface; 
 a radiation emitting source ( 201 ) arranged to illuminate the earth formation ( 16 ) surrounding the borehole; 
 at least one radiation detector ( 211 ) arranged to detect radiation reflected by the earth formation resulting from illumination by the source ( 201 ); 
 at least one source collimation window ( 202 ) and one detector collimation window ( 212 ) through which the earth formation is illuminated and radiation is detected; and 
 at least one radiation shield ( 30 ) located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic that has not traversed the outer collar and is located between said emitting radiation source and said radiation detector, said radiation shield having a length along the axis is less than 80% of the distance between said emitting radiation source ( 201 ) and said radiation detector ( 211 ). 
 
 
     
     
       12. A logging tool ( 20 ) for underground formations surrounding a borehole ( 14 ), comprising:
 an elongated body ( 21 ) along a major axis; 
 a collar ( 22 ) disposed peripherally around said body ( 21 ) having a collar wall defined by an inner and an outer surface; 
 a radiation emitting source ( 201 ) arranged to illuminate the earth formation ( 16 ) surrounding the borehole; 
 at least one radiation detecttor ( 211 ) arranged to detect radiation reflected by the earth formation resulting from illumination by the source ( 201 ); 
 at least one source collimation window ( 202 ) and one detector collimation window ( 212 ) through which the earth formation is illuminated and radiation is detected; 
 at least one radiation shield located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic radiation that has not traversed the outer collar; and 
 another radiation shield ( 31 ) having an annular shape surrounding the detector collimation window ( 212 ).